In order to create a dropset, simply use the swipe right gesture in the set you wish to convert. It only works on a normal (non-warmup) set that already has its reps and weight filled in. Each swipe adds one drop, so swipe twice for a two-drop chain, and a third swipe clears them.
You can also open the exercise's 3-dot menu and tap "Add dropset" to turn its last set into a dropset, or "Remove dropset" to undo it. And if you want Gravl to add dropsets to your generated workouts on its own, turn it on in Workout Settings > Optional > Dropsets.